-
HEAT TRANSFER AND
CONDENSATION OF WATER
VAPOUR FROM HUMID AIR IN
COMPACT HEAT EXCHANGERS
Mohammad Saraireh
A thesis submitted in fulfilment of the requirements of the
Degree of
Doctor of Philosophy
at
School of Engineering and Science
Faculty of Health, Engineering and Science
Victoria University
Melbourne, Australia
February 2012
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ABSTRACT
In this thesis, an experimental and simulation study of heat
transfer in water-to-air
compact-plate heat exchanger is presented. A compact-plate heat
exchanger made of
polypropylene, in which flow pattern is maintained as
counter-current, was
constructed to conduct the experiments. Experiments were
conducted for different
operation conditions of hot and cold fluids, in which hot water
is considered hot
streams and air is considered cold streams. The thermal
performance of the plate heat
exchanger was analysed using the experimental data.
Computational fluid dynamics
(CFD) package FLUENT® was used to predict the fluid flow and
heat transfer in the
plate heat exchanger and to study the transient response of the
system to changes of
inlet temperature for both fluids. The results of the heat
rejection rate are presented
for the heat exchanger, which is simulated according to the
configuration of the plate
heat exchanger used in the experiments. The model was also
simulated at different
operation conditions and compared with experimental data. The
simulated results are
in good agreement with experimental results.
When the heat exchangers are used as condensers, vapour
condensation occurs in the
heat exchangers. To enhance our physical understanding of
condensation, a new
mathematical model for the condensation of water vapour from
humid air in a heat
exchanger is presented. The mathematical model is developed
using heat and mass
balances in a plate heat exchanger in which a mixture of water
vapour and non-
condensable gas (air) is cooled by liquid water. Numerical
predictions of
condensation rate, heat flux and outlet water temperature in the
plate heat exchanger
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are compared with experimental results from the literature and
good agreement is
found. The model is also used for tube condensers to predict the
condensation rate, the
bulk temperatures of the coolant and the gas-vapour mixture, and
the inside and
outside surface temperatures of the condenser wall. The
predicted results for the
counter-flow tube condensers are compared with three sets of
published experimental
data. The results from this study show that when modelling
vapour condensation in
the presence of non-condensable gas, a simple model for the
mixture channel alone is
not sufficient, since neither the temperature nor the heat flux
at the wall separating the
mixture and the coolant can be assumed constant. The results
also show that the wall
temperature in the coolant channel can be quite high because of
the resistance of heat
transfer, and careful modelling of the heat transfer in the
coolant channel is needed to
achieve good agreement between the model predictions and the
experimental results.
Finally, CFD simulations of heat and mass transfer involving the
condensation of
vapour in the presence of non-condensable gases in plate heat
exchanger are
presented. The simulations were carried out using FLUENT®.
Convective heat and
mass transfer and vapour condensation at a constant wall
temperature are investigated
for a plane channel with the aim of comparing the CFD results
with well-established
correlations. CFD simulations of heat and mass transfer and
water-vapour
condensation in the presence of non-condensable air are then
carried out for constant
heat transfer coefficients for the condensation wall and coolant
with different mass
fractions of water vapour and inlet velocities. The predictions
obtained from this are
compared with experimental data and reasonable agreement is
found for the
condensation rates of water vapour and heat flux. Finally, the
condensation of the
water vapour is simulated in a heat exchanger including both the
cooling water and
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vapour-air mixture channels separated by solid walls. This
simulation is unique and
close to reality and no assumptions are required for the
temperature or heat transfer
coefficient at the condensing wall. The difficulties of
simultaneously simulating a gas
mixture and liquid flowing in separate channels using
commercially available CFD
software are discussed and strategies to overcome these
difficulties are outlined.
Results from this simulation are presented and compared with
available experimental
results.
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DECLARATION
I, Mohammad Saraireh, declare that the PhD thesis entitled ‘Heat
transfer and
condensation of water vapour from humid air in compact heat
exchangers’ is no more
than 100,000 words in length including quotations and exclusive
of tables, figures,
appendices, bibliography, references and footnotes. This thesis
contains no material
that has been submitted previously, in whole or in part, for the
award of any other
academic degree or diploma. Except where otherwise indicated,
this thesis is my own
work.
Signature: Date: 29/08/2012
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ACKNOWLEDGEMENTS
I would like to take this opportunity to acknowledge those who
have supported me
during my candidature. Firstly, I would like to thank my
supervisors, Associate
Professor Jun-De Li and Professor Graham Thorpe, for giving me
this opportunity, for
their constant support and encouragement throughout, and for
their belief that I would
complete my candidature.
I would also like to thank my colleagues and all the members of
the School of
Engineering and Science at Victoria University of Technology for
their help and
support.
Finally, I wish to thank my family. My appreciation extends to
my parents for their
encouragement and support, and to my brothers and sister.
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LIST OF PUBLICATIONS
Conference papers
Saraireh, M, Li, JD & Thorpe, GR 2010, ‘Modelling of heat
and mass transfer
involving vapour condensation in the presence of non-condensable
gases’,
17th Australasian fluid mechanics conference, Auckland, New
Zealand.
Saraireh, M, Thorpe, G & Li, J-D 2011, ‘Simulation of heat
and mass transfer
involving vapour condensation in the presence of non-condensable
gases in
plane channels’, ASME conference proceedings,
T10026-T10026-10.
Journal paper
Li, J-D, Saraireh, M & Thorpe, G 2011, ‘Condensation of
vapour in the presence of
non-condensable gas in condensers’, International Journal of
Heat and Mass
Transfer, vol. 54, pp. 4078–4089.
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NOMENCLATURE
mB dimensionless driving force based on mass fractions
pC specific heat 1 1( )J kg K− −
C concentration 3( )kg m−
d diameter of the inner pipe ( )m
D diffusivity 2 1( )m s−
i ,mD diffusion coefficient for species i in the mixture 2 1( )m
s−
,T iD thermal diffusion coefficient 2 1( )m s−
E energy ( )J
f Fanning friction factor
g acceleration due to gravity 2( )m s−
bG generation of turbulence kinetic energy due to buoyancy
kG generation of turbulence kinetic energy due to mean velocity
gradients
G mixture mass flux 2 1( )kg m s− −
Gr Grashof number
H heat transfer coefficient 2 1( )W m K− −
hv specific enthalpy of the vapour 1( )J kg −
hf specific enthalpy of the condensate 1( )J kg −
,fg ih latent heat of water vapour at the interface 1( )J kg
−
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iJr
diffusion flux of species i 2 1( )kg m s− −
k thermal conductivity 1 1( )W m K− −
k turbulence energy 2 2( )m s −
K mass transfer coefficient 1( )m s−
L length of the pipe ( )m
M molecular weight
m& mass flow rate 1( )kg s−
condm& mass condensation rate, 1( )kg s−
m ′′& mass flux 2 1( )kg m s− −
Nu Nusselt number
Pr Prandtl number pC
Prkμ⎛ ⎞
=⎜ ⎟⎝ ⎠
P pressure ( )Pa
vP saturation vapour pressure ( )Pa
"q heat flux 2( )W m−
sQ sensible heat ( )W
lQ latent heat ( )W
Ra Rayleigh number
Re Reynolds number
iR net rate of production of species i
ir tube inner radius ( )m
or condenser tube outer radius ( )m
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Sc Schmidt number
tSc turbulent Schmidt number
Sh Sherwood number
, kS S ε source terms in Equations 5.1 and 5.2, respectively
iS source term in Equation 5.3
hS volumetric heat source ( )3W m −
t time ( )s
T temperature ( )Cο
,out wT outlet water temperature ( )Cο
,in wT inlet water temperature ( )Cο
T∞ dry bulb temperature ( )Cο
U bulk velocity 1( )m s−
, ,u v w velocity components 1( )m s−
V velocity 1( )m s−
x distance from the inlet ( )m
0x initial entrance length ( )m
Y mass fraction
y lateral position ( )m
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Greek symbols
β thermal expansion coefficient
α thermal diffusivity 2 1( )m s−
, k εα α inverse effective Prandtl numbers for k andε ,
respectively
δ thickness of condensate film ( )m
sδ thickness of solid wall ( )m
ε roughness height ( )m
ε dissipation in Equations 5.1 and 5.2 ( )2 3m s −
μ dynamic viscosity 1 1( )kg m s− −
tμ turbulent viscosity 1 1( )kg m s− −
effμ effective viscosity 1 1( )kg m s− −
ρ density 3( )kg m−
ϖ specific humidity
gτ interfacial shear stress 2( )N m−
τ stress tensor 2( )N m−
Subscripts
a air
c condensate
,f i film interface
f film
g gas
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,i j species
l liquid
o without suction
r roughness
s solid, smooth
t developing
v vapour
av air-vapour
,v i water vapour at the air-liquid interface
,v b water vapour in the air-vapour mixture
w water
,w w at the wall
wb wet bulb
Abbreviations
CFD computational fluid dynamics
HTC heat transfer coefficient
UDF user-defined functions
UDS user-defined scalar
UJV ustav jaderneho vyzkumu (Nuclear Research Institute)
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TABLE OF CONTENTS
Abstract
..........................................................................................................................
i
Declaration...................................................................................................................
iv
Acknowledgements
......................................................................................................
v
List of Publications
.....................................................................................................
vi
Nomenclature
.............................................................................................................
vii
Table of Contents
.......................................................................................................
xii
List of Figures
...........................................................................................................
xvii
List of Tables
...........................................................................................................
xxiv
Chapter 1: Introduction
..............................................................................................
1
1.1. Introduction
.........................................................................................................
1
1.2. Research Objectives
............................................................................................
3
1.3. Thesis Layout
......................................................................................................
4
Chapter 2: Literature Review
.....................................................................................
5
2.1. Polymer Heat Exchangers
...................................................................................
5
2.2. Theoretical Studies of Condensation
..................................................................
7
2.3. Experimental Studies of Condensation
...............................................................
9
2.4. Heat and Mass Transfer with Condensation
..................................................... 11
2.5. Simulation of Condensation Using CFD
..........................................................
14
2.6. Conclusion
........................................................................................................
16
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xiii
Chapter 3: Experimental and Numerical Study of Heat Transfer in
Compact
Heat Exchangers
........................................................................................................
18
3.1. Introduction
.......................................................................................................
18
3.2. The Compact Heat Exchanger
..........................................................................
19
3.2.1. Experimental Setup
....................................................................................
21
3.2.2. Water and Air Supply System
....................................................................
22
3.2.3. Measurement System
..................................................................................
24
3.3. Experiment Procedure
.......................................................................................
26
3.4. Data Reduction
.................................................................................................
27
3.5. CFD Simulation
................................................................................................
29
3.5.1. The Governing Equations
...........................................................................
30
3.5.1.1. The Continuity Equation
....................................................................
30
3.5.1.2. The Momentum Equation
..................................................................
31
3.5.1.3. The Energy Equation
.........................................................................
31
3.5.2. Geometry Creation
.....................................................................................
33
3.5.3. Boundary Conditions
..................................................................................
35
3.5.3.1. Inlet Boundary Conditions
................................................................
35
3.5.3.2. Outlet Boundary Conditions
..............................................................
35
3.5.3.3. Wall Boundary Conditions
................................................................
35
3.5.3.4. Periodic Boundary Conditions
..........................................................
36
3.5.3.5. Symmetry Boundary Conditions
........................................................
36
3.5.4. Grid Independence and Meshing
................................................................
37
3.5.5. Working Fluid Properties
...........................................................................
38
3.5.6. Solid Properties
..........................................................................................
38
3.5.7. Solver
..........................................................................................................
39
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xiv
3.6. Results and Discussion
.....................................................................................
39
3.6.1. Experimental Results
..................................................................................
39
3.6.2. Simulation Results
......................................................................................
44
3.6.2.1. Flow Visualisation
.............................................................................
44
3.6.2.2. Temperature Distribution
..................................................................
46
3.6.2.3. Velocity Distribution
.........................................................................
48
3.7. Validation of Simulation Results with Experimental Results
.......................... 49
3.8. Comparison of the Heat Rejection Rate between Aluminium
Alloy and
Polypropylene Compact Heat Exchanger
...........................................................
51
3.9. Transient Response of the Polypropylene Plate Compact Heat
Exchanger
to Inlet Temperature Change
...............................................................................
52
3.10. Conclusions
.....................................................................................................
54
Chapter 4: Mathematical Model of Condensing Heat Exchanger
........................ 56
4.1. Mathematical Model of Heat Exchanger with Condensation
........................... 56
4.1.1. Heat and Mass Transfer Analysis
...............................................................
58
4.1.1.1. Heat Balance on the Air-Vapour Mixture
......................................... 58
4.1.1.2. Force and Momentum Balance on the Condensate Film
.................. 60
4.1.1.3. Heat Transfer at the Solid Wall
.........................................................
62
4.1.1.4. Heat Transfer in the Cooling Channel
.............................................. 62
4.2. Mathematical Model of Tube Condenser
.........................................................
63
4.3. Models and Formulations of Heat and Mass Transfer
...................................... 66
4.3.2. Suction Effect Consideration
......................................................................
68
4.3.3. Developing Flow Consideration
.................................................................
70
4.3.4. Roughness Effect Consideration
................................................................
71
4.3.5. Thermophysical Properties
.........................................................................
72
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4.3.6. Buoyancy Effect
.........................................................................................
73
4.4. Numerical Solution Procedures
........................................................................
74
4.5. Results and Discussion
.....................................................................................
79
4.5.1. Comparison of Model Predictions for Condensation in Plate
Heat
Exchanger with Existing Experimental Results
.......................................... 79
4.5.2. Comparison of Model Predictions for Condensation in Tube
with
Existing Experimental Results
....................................................................
83
4.5.2.1. Condensation Rate
.............................................................................
86
4.5.2.2. Temperature
......................................................................................
89
4.6. Discussion and Conclusions
...........................................................................
102
Chapter 5: CFD Simulation of Condensing Heat Exchanger
.............................. 105
5.1. Governing Equations and Turbulence Modelling
........................................... 105
5.1.1. Turbulence Equations
...............................................................................
105
5.1.2. Species Transport Equations
....................................................................
106
5.2. Simulation of Condensation
............................................................................
107
5.2.1. The Experiments of Ambrosini et al. (2008) and the
Benchmark Tests .. 107
5.2.1.1. Step 0
...............................................................................................
110
5.2.1.2. Step 1
...............................................................................................
118
5.2.1.3. Step 2
...............................................................................................
120
5.3. Conclusions and Discussion
...........................................................................
126
Chapter 6: Conclusions and Recommendations
................................................... 128
6.1. Conclusions
.....................................................................................................
128
6.2. Recommendations
...........................................................................................
131
References
.................................................................................................................
132
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xvi
Appendices
................................................................................................................
143
Appendix A: Tables of Experimental Data
............................................................
143
Appendix B: Thermophysical Properties of Fluids
............................................... 148
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xvii
LIST OF FIGURES
Figure 3.1: Schematic of the plate heat exchanger.
..................................................... 20
Figure 3.2: Photograph of the compact-plate heat exchanger.
..................................... 21
Figure 3.3: Photograph of the experimental setup for
water-to-air heat exchanger
test.
..............................................................................................................
22
Figure 3.4: XF 192S pump for circulating water.
........................................................
23
Figure 3.5: Speed controller.
........................................................................................
24
Figure 3.6: The data acquisition system.
.....................................................................
25
Figure 3.7: Schematic of the inputs required in FLUENT® and some
outputs. ........... 30
Figure 3.8: Plate heat exchange geometry.
..................................................................
33
Figure 3.9: Domain dimensions and boundary conditions used in
this study. ............ 34
Figure 3.10: Grid independence test.
...........................................................................
37
Figure 3.11: Comparison of the heat rejection rate at various
air and water mass
flow rates (results from Victoria University laboratory).
............................ 40
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xviii
Figure 3.12: Temperature of water entering and leaving the
compact heat
exchanger and the ambient air temperature for the period 24 to
25
January 2011 (results from Yallourn power
station)................................... 41
Figure 3.13: Heat rejected by the polypropylene compact heat
exchanger for the
period 24 to 25 January 2011.
.....................................................................
42
Figure 3.14: Temperature of water entering and leaving the
compact heat
exchanger and the ambient air temperature for the period 15 to
24
February 2011.
............................................................................................
43
Figure 3.15: Heat rejected by the compact heat exchanger for the
period 15 to 24
February 2011.
............................................................................................
43
Figure 3.16: Flow visualisation inside the plate heat exchanger
that indicates the
decrease in temperature in the direction of flow.
........................................ 44
Figure 3.17: Flow visualisation inside the plate heat exchange
for air channel that
indicates the increase in temperature in the direction of flow.
................... 45
Figure 3.18: Temperature distribution inside the water channel.
................................ 46
Figure 3.19: Temperature distribution over plate surface of the
plate heat
exchanger.
...................................................................................................
47
Figure 3.20: Velocity streamline inside the water channel. Water
enters at the top
left-hand corner and leaves at the bottom left-hand
corner......................... 48
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xix
Figure 3.21: Comparison of the measured and simulated values of
the heat
rejection rates for the compact heat exchanger at water flow
rate of
0.25 kg/s.
.....................................................................................................
49
Figure 3.22: Comparison of the measured and simulated values of
the heat
rejection rates for the compact heat exchanger for the period 24
to 25
January 2011.
..............................................................................................
50
Figure 3.23: Comparison of the measured and simulated values of
the heat
rejection rates for the period 25 to 27 January 2011.
.................................. 50
Figure 3.24: Transient response of water stream.
........................................................
53
Figure 3.25: Transient response of air stream.
.............................................................
54
Figure 4.1: Schematic diagram of the small element in
considering control
volume analysis.
..........................................................................................
57
Figure 4.2: Schematic diagram of a tube condenser.
................................................... 64
Figure 4.3: Comparison of the predicted and experimental heat
flux for all the
tests of Ambrosini et al. (2008).
..................................................................
81
Figure 4.4: Comparison of the predicted and experimental values
of condensation
rate for all the tests of Ambrosini et al. (2008).
.......................................... 82
Figure 4.5: Comparison of the predicted and experimental values
of outlet water
temperature Tw,out for all the tests of Ambrosini et al. (2008).
.................... 83
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xx
Figure 4.6: Comparison of the predicted condensation rates with
the experimental
results of Siddique (1992), runs 1, 6, 7, 11, 13 and 17.
.............................. 87
Figure 4.7: Comparison of the predicted condensation rates with
the experimental
results of Kuhn (1995), runs 2.1-12, 2.2-1, 2.2-8, 3.1-2, 3.3-4,
3.5-4,
4.5-3 and 4.5-5.
...........................................................................................
88
Figure 4.8: Comparison of the predicted condensation rates with
the experimental
results of Tanrikut and Yesin (1998), runs 2.3.1, 3.3.1, 3.4.1,
4.3.1 and
5.4.1.
............................................................................................................
88
Figure 4.9: Comparison of the predicted temperatures of cooling
water, wall and
air-vapour mixture with the experimental results of Siddique
(1992),
run 7.
...........................................................................................................
90
Figure 4.10: Comparison of the predicted temperatures of cooling
water, wall and
air-vapour mixture with those measured by Siddique (1992), run
11......... 90
Figure 4.11: Comparison of the predicted temperatures of cooling
water, water
and air-vapour mixture with those measured by Siddique (1992),
run
17.
................................................................................................................
91
Figure 4.12: Comparison of the predicted temperatures of cooling
water, water
and air-vapour mixture with those measured by Siddique (1992),
run
26.
................................................................................................................
91
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xxi
Figure 4.13: Comparison of the predicted heat flux with the
experimental results
of Siddique (1992), run 26.
.........................................................................
93
Figure 4.14: Comparison of the predicted temperatures of cooling
water, air-
vapour mixture, inner wall and outer wall with the experimental
results
of Kuhn (1995), run 2.1-12.
........................................................................
95
Figure 4.15: Comparison of the predicted temperatures of cooling
water, air-
vapour mixture, inner wall and outer wall with the experimental
results
of Kuhn (1995), run 2.2-8.
..........................................................................
96
Figure 4.16: Comparison of the predicted temperatures of cooling
water, air-
vapour mixture, inner wall and outer wall with the experimental
results
of Kuhn (1995), run 3.5-4.
..........................................................................
96
Figure 4.17: Comparison of the predicted temperatures of cooling
water, mixture,
inner wall and outer wall with the experimental results of Kuhn
(1995),
run 4.5-5.
.....................................................................................................
97
Figure 4.18: Comparison of predicted heat flux with the
experiment results of
Kuhn (1995), run 2.1-12.
............................................................................
98
Figure 4.19: Comparison of the predicted temperatures of cooling
water, wall and
air-vapour mixture with the experimental results of Tanrikut and
Yesin
(1998), run 2.3.1.
.........................................................................................
99
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xxii
Figure 4.20: Comparison of the predicted temperatures of cooling
water, wall and
air vapour mixture with the experimental results of Tanrikut and
Yesin
(1998), run 3.3.1.
.........................................................................................
99
Figure 4.21: Comparison of the predicted temperatures of cooling
water, wall and
air-vapour mixture with the experimental results of Tanrikut and
Yesin
(1998), run 3.4.1.
.......................................................................................
100
Figure 4.22: Comparison of the predicted temperatures of cooling
water, wall and
air-vapour mixture with the experimental results of Tanrikut and
Yesin
(1998), run 5.4.1.
.......................................................................................
100
Figure 5.1: Experimental test section (Ambrosini et al. 2008).
................................. 108
Figure 5.2: Computational domain and grid for steps 0 and 1.
.................................. 110
Figure 5.3: Grid independence test.
...........................................................................
111
Figure 5.4: Results obtained for two heat transfer cases in step
0: (A) T30-V3,
(B) T30-V6.
...............................................................................................
116
Figure 5.5: Results obtained for two heat and mass transfer
cases in step 0: (a)
T30-V3, (b) T30-V6.
.................................................................................
117
Figure 5.6: Comparison of calculated and experimental values of
heat flux. ............ 119
Figure 5.7: Comparison of calculated and experimental values of
condensation
rate.
............................................................................................................
119
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xxiii
Figure 5.8: Computational domain for step 2.
...........................................................
121
Figure 5.9: Comparison of calculated and experimental values of
condensation
rate from step 2.
........................................................................................
125
Figure 5.10: Comparison of calculated and experimental values of
heat flux from
step 2.
........................................................................................................
126
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xxiv
LIST OF TABLES
Table 3.1: Test
runs......................................................................................................
27
Table 3.2: Comparison of the heat rejection rate between
Polypropylene and
Aluminium heat exchanger.
........................................................................
52
Table 4.1: Experimental conditions from Ambrosini et al. (2008).
............................. 80
Table 4.2: Experimental runs used for comparison with
predictions. ......................... 84
Table 5.1. Experimental conditions of tests 1–5 conducted by
Ambrosini et al.
(2008).
.......................................................................................................
109
-
Chapter 1: Introduction 1
CHAPTER 1: INTRODUCTION
1.1. Introduction
In this thesis, the research includes three major parts: an
experimental and simulation
study of heat transfer in polypropylene compact-plate heat
exchanger; a mathematical
model of condensation of water vapour from humid air in plate
and tube condensers;
and a simulation study of heat and mass transfer in vapour
condensation in the
presence of non-condensable gas using computational fluid
dynamics (CFD) software
FLUENT® for plate condensers.
Heat exchangers are used to transfer heat from one fluid to
another in such diverse
applications as refrigeration, ventilation and air-conditioning
systems, power
generation, the manufacturing and space industries, and
environmental engineering.
Although there are a wide variety of heat exchangers in the
market, the present work
focuses on counter-current polymer compact-plate heat
exchangers. Their excellent
heat transfer characteristics, easy fabrication and maintenance,
low weight, excellent
corrosion resistance and cost competitiveness has made the
polymer compact heat
exchanger an important choice for engineering applications.
Thus, the present thesis
begins by investigating the experiments of heat transfer in a
compact-plate heat
exchanger made of polypropylene for water-to-air heat
exchange.
-
Chapter 1: Introduction 2
When the heat exchangers are used as condensers, vapour
condensation can occur in
the heat exchangers. Condensation plays a key role in systems
such as air
conditioning, power plants, refrigeration, reactor safety,
aerospace and desalination.
Environmental and economic pressures are driving the need to
design increasingly
efficient systems. As a result, we need to increase our
understanding of condensation.
Condensation is initiated and sustained when the temperature of
a surface is
maintained below the dew-point temperature of the surrounding
vapour. In the
process, a condensate is formed with the liberation of latent
heat. The process may
take place in one of two modes, namely film condensation or
drop-wise condensation.
In film condensation, the surface is completely wetted by the
condensate and is
blanketed by a liquid film of increasing thickness as it flows
downward under the
influence of gravity. Conversely, in drop-wise condensation, the
surface is not fully
wetted by the condensate and droplets slide downward after they
attain a certain size.
It is difficult to sustain drop-wise condensation, which may be
achieved initially on
some surfaces, and the process usually converts to film
condensation after some time.
Therefore, it is customary to assume film condensation in the
design of condensers
(Bum-Jin et al. 2004).
Computational modelling, such as that performed by CFD software
FLUENT®, has
received much attention in recent years, becoming increasingly
popular as an
alternative approach to cope with real-world problems.
Computational modelling can
provide detailed information about the fluid flow and heat and
mass transfer
mechanism. Further, numerical methods are more flexible and much
cheaper, as they
give the opportunity for testing new methods before they are
executed through
-
Chapter 1: Introduction 3
experiments, which are often costly. Thus, it is important to
use this technology in the
modelling and simulation of heat transfer and condensation in
heat exchangers.
1.2. Research Objectives
The first objective of the present work is to introduce an
experiment and numerical
simulation of heat transfer in a counter-current polypropylene
compact-plate heat
exchanger for water-to-air heat exchange. To achieve this, a
compact heat exchanger
was fabricated and experiments were set up to examine the heat
rejection rate in the
heat exchanger. The validation of the experiment results with
the simulation results
from CFD is presented.
The second objective of this research is to study the
condensation of water vapour
from humid air in a heat exchanger. To this end, a mathematical
model of air-vapour
mixture to water heat exchanger, taking into account
condensation of water vapour, is
formulated. Such a model allows for a better understanding of
the condensation of
water vapour and the coupling of heat and mass transfer.
CFD is used to simulate the condensation problem in a heat
exchanger. The
simulation examines the process of heat and mass transfer that
is involved in
condensation. The results from the mathematical model and CFD
software are
validated with the available experimental data from the
literature.
-
Chapter 1: Introduction 4
1.3. Thesis Layout
This thesis is divided into the following chapters:
Chapter Two
This chapter contains a review of previous work of polymer heat
exchangers, and
theoretical and experimental studies of condensation.
Chapter Three
This chapter presents the experimental and simulation study of
heat transfer in a
water-to-air counter-flow polypropylene compact-plate heat
exchanger.
Chapter Four
A new mathematical model of condensation in plate and tube
condensers is
introduced in this chapter, and a comparison between the present
work and
experimental data reported in the literature is presented.
Chapter Five
A CFD simulation and results of condensation in a plate heat
exchanger are presented.
Chapter Six
This chapter presents the conclusions of the research and makes
recommendations for
future work.
-
Chapter 2: Literature review 5
CHAPTER 2: LITERATURE REVIEW
2.1. Polymer Heat Exchangers
Using polymer heat exchangers to transfer thermal energy has
received considerable
attention from researchers. Perry, Dietz and Shannon (1983) used
thin polymer film
heat exchangers for large-scale operations, such as in
converting saline or salty water to
pure water. Schnon (1988) designed a plastic film plate heat
exchanger for the recovery
or dissipation of heat energy in buildings. Polymer heat
exchangers can also be used for
heat recovery in the chemical process, or in electrical power
and other industries. This
type of heat exchanger is designed for operation at low absolute
pressures, for example,
from 1370 kPa to 2060 kPa maximum.
Several polymeric materials for use in condensing heat
exchangers attached to high
efficiency gas fired furnaces were studied by Bigg, Stickford
and Talbert (1989).
Fluorinated ethylene propylene shrink film coverings, glass
fibre reinforced poly
(phenylene sulphide) extruded tubes and poly (ether imide) tubes
were some of the
materials that were studied. These polymers showed no evidence
of degradation after
10,000 cycles in a gas fired condensing heat exchanger. Heat
transfer and pressure drop
in gas-to-air plastic heat exchanges was studied by Hetsroni and
Mosyak (1994).
Polymer materials such as polyethylene, polyester and poly
vinylidene chloride were
tested and heat transfer coefficients (HTCs) of 80–90 W m-2 K-1
were reported.
-
Chapter 2: Literature review 6
Another study on heat transfer and pressure drop in air-to-air
and water-to-water
compact cross-flow plate heat exchangers made of poly (ether
ether ketone) (PEEK)
was conducted by Jachuck and Ranshaw (1994). They recognised
that this new
generation of polymers could play a key role in the development
of a new class of
compact heat exchangers, which could be cheaper, lighter and
more corrosion resistance
than metal heat exchangers. Air-to-water heat transfer
experiments in cross-flow plate
heat exchangers made of polypropylene were described by Brouwers
and Van der Geld
(1996), who reported heat transfer coefficient of around 55 W
m-2 K-1.
A plastic heat exchanger made from polypropylene (PP) and
polyvinylidene fluoride
(PVDF) was constructed by Davidson et al. (1999) to produce
solar collectors for
heating swimming pools. They concluded that the maximum pressure
of the PP heat
exchanger was 200 kPa at 80°C and that the PVDF heat exchanger
could withstand 600
kPa at 100°C. Polymer plate heat exchanger made from PVDF and PP
is used
extensively in heating and ventilation systems, such as in car
radiators. It is a form of
constructed prototype plate exchanger using 60 plates measuring
45cm × 45cm. The
unit transfer 26.4kW heat and it operates successfully at 75°C
with 6.5 bar differential
pressure (Reay, 2000).
The use of polymeric hollow fibre heat exchangers as a new type
of heat exchanger for
lower temperature and pressure applications was proposed by
Zarkadas and Sirkar
(2004). They recognised that the polymeric materials could offer
many advantages over
metals in the construction of heat exchangers. First, they are
less expensive and easier to
-
Chapter 2: Literature review 7
shape than metals. Further, the energy required to produce a
unit mass of plastics is
lower than that of metals, making them environmentally
attractive.
Polymer film compact heat exchangers (PFCHE) made from PEEK for
use in the fuel-
cell industry were studied by Zaheed and Jachuck (2005). They
concluded that using
PFCHE brings several benefits over the metallic fuel-cell
designs in terms of overall
performance, energy and cost saving.
2.2. Theoretical Studies of Condensation
The field of condensation owes its current state to the
pioneering work of Nusselt
(1916), who predicted, from a simplified theoretical analysis,
the HTC of stationary
pure vapour in film condensation on a vertical flat plate.
Improvements and
modifications to Nusselt’s theoretical solution have been made
by a number of
researchers. For example, Bromley (1952) assumed a linear
temperature distribution in
the liquid film model. Subsequently, Rohsenow (1973) and others
considered the effect
of interfacial shear stress on both condensation flow and the
characteristic of vapour
velocity diminishing along the length of a tube. These
investigations were concerned
with condensation of pure vapour.
In many industrial operations, some amount of non-condensable
gas may exist in
vapours. It was well recognised that the presence of
non-condensable gas in vapours
could greatly reduce condensation heat transfer and deteriorate
the performance of
devices. Seminal studies on this topic were conducted by Sparrow
and Lin (1964),
-
Chapter 2: Literature review 8
Minkowycz and Sparrow (1966) and Sparrow, Minkowycz and Saddy
(1967). They
explored analytically the condensation of gases containing
non-condensable gases in
forced convection flow along a horizontal flat plate. Following
their studies, a number
of publications were devoted to the condensation of gas mixtures
in laminar or turbulent
flow along horizontal, vertical or inclined plates, inside or
outside tubes and in tube
bundles.
For example, Patankar and Sparrow (1979) studied film
condensation in the presence of
non-condensable gas on a vertical fin that was attached to a
cooled vertical plate or
cylinder. A physically meaningful similarity solution of the
problem was formulated
and numerically evaluated, and a finite difference marching
solution of the governing
equations for conjugate problem was performed. Further, Webb and
Wanniarachchi
(1980) developed a one-dimensional numerical model to predict
the effect of non-
condensable gases in a 10-row by 10-column finned tube heat
exchanger by solving the
Colburn and Hougen (1934) equation for refrigerant R-11 and air
mixture. An iterative
solution procedure was applied to solve the equation. The
modelling results were not
verified with measured data.
A solution showing the effects of a non-condensable gas on the
film-wise condensation
of a vapour-gas mixture with turbulent flow in a vertical tube
using the heat and mass
transfer analogy was presented by Wang and Tu (1988). They found
that the reduction
in heat transfer due to the non-condensable gas was more
significant at low pressures
and in mixtures flowing with low Reynolds numbers.
-
Chapter 2: Literature review 9
Karapantsios, Kostoglou and Karabelas (1995) measured local
condensation rates
during the direct-contact condensation of water vapour from a
mixture with air. The
influence of the film Reynolds number was discussed. They found
that an increase in
the film Reynolds number increases the condensation HTC.
However, with the presence
of non-condensable gas, the condensation HTC decreases. The
effect of the presence of
a non-condensable gas, such as air on condensation rate for
external condensation for
the cases of stagnant as well as flowing vapour, was
theoretically investigated by
Dharma Rao et al. (2002).
2.3. Experimental Studies of Condensation
Condensation has been experimentally studied by many
researchers. Lebedev,
Baklastov and Sergazin (1969) performed an experimental study of
combined heat and
mass transfer in the condensation of vapour from humid air on a
flat plate. They found
that the condensation heat transfer increases with the relative
humidity and the velocity
of the air.
An experimental study of condensation in vertical tubes was
performed by Siddique
(1992), who used the experimental apparatus of an open cooling
water circuit and an
open non-condensable gas/steam loop for forced convection
condition. The condenser
tube dimensions were 50.8 mm OD, 46.0 mm ID and 2.54 m effective
length. They
concluded that the local HTC depends strongly on the mixture
Reynolds number,
increasing as the Reynolds number increases.
-
Chapter 2: Literature review 10
Kim and Kang (1993) and Karapantsios et al. (1995) measured the
condensation heat
transfer rate for steam–air mixtures in direct contact with the
sub-cooled water layers.
The condensation HTCs were found to be dependent not only on the
steam
concentration but also on the wave characteristics of the
falling liquid layer.
Condensation of vapours in the presence of non-condense gas on a
short horizontal tube
was studied by Mamyoda and Asano (1994). They compared their
analysis with their
own experimental data. Satisfactory agreement was obtained
between their experimental
results and a mathematical model. Kuhn (1995) performed an
experimental study of
condensation by using a 3.37 m long seamless stainless steel
tube with a 50.8 mm OD
and 1.65 mm wall thickness for steam/air and steam/helium.
Experiments with gas-vapour mixtures in the presence of
non-condensable gas were
conducted by Ganzevles and Van der Geld (2002) to study the
importance of the
Marangoni effect in actual condensation processes for a compact
polymer heat
exchanger. The heat resistances were taken into consideration
and the heat resistance of
the condensate was quantified. The study introduces Nusselt and
Sherwood numbers
and friction factors based on a characteristic length scale
valid in drop-wise
condensation. New correlations were obtained from the
experiments with a variety of
process conditions.
An experimental study was conducted by Tanrikut and Yesin (2005)
for steam
condensation in the presence of air flowing downward in a
vertical tube of 2.15 m with
33/39 mm ID/OD, and cold water flowing upward inside the jacket
pipe of 2.133 m and
-
Chapter 2: Literature review 11
81.2/89 mm ID/OD. They presented both the temperature profile of
the coolant in the
jacket pipe and the mixture temperature in the condenser
tube.
The use of condensing heat exchangers to recover water vapour
from flue gas at coal-
fired power plants was studied by Levy et al. (2008b, 2008a).
Pilot scale heat transfer
tests were performed to determine the relationship between
flue-gas moisture
concentration, heat exchanger design and operating conditions,
and water-vapour
condensation rate.
2.4. Heat and Mass Transfer with Condensation
The analysis by heat and mass transfer analogy in situations
with condensation has been
described by many researchers. Colburn and Hougen (1934) were
the first to develop a
theory for condensation mass transfer that was controlled by the
mass concentration
gradient through non-condensable layer. They described the heat
transfer process as the
sum of sensible heat and latent heat flows. Much later,
Corradini (1984) added a
correction factor that accounts for the suction effect at high
mass transfer rates across
the liquid-gas interface. Then, Kim and Corradini (1990)
incorporated the effects of film
roughness on the gas phase heat and mass transfer for a flat
plate.
An analytical study of condensation in a vertical tube using the
analogy between heat
and mass transfer was conducted by Siddique, Golay and Kazimi
(1994). Their model
included the effects of developing flow, condensate film
roughness, suction and
property variation in the gas phase. Pele, Baudoin and Barrand
(1994) studied the effect
-
Chapter 2: Literature review 12
of the flow rate of saturated humid air on the laminar film-wise
condensation inside a
vertical cooled pipe during turbulent forced convection. They
found that the local
condensate HTC decreases along the length of the pipe.
Heat and mass transfer of air-water-vapour mixture in a cross
flow heat exchanger was
investigated theoretically and experimentally by Brouwers and
Van der Geld (1996).
From comparing the results, it was found that the heat exchanger
model could predict
the rate of heat transfer and condensation phenomenon. A
theoretical prediction of heat
and mass transfer in a vertical tube condenser from steam and
non-condensable gas
mixture was derived by Dehbi and Guentay (1997). An algebraic
equation for the film
thickness was derived. Then, mass and heat transfer analogy were
invoked to deduce the
condensation rate.
According to Takarada et al. (1997), the analogy between the
heat and mass transfer is
applicable at low concentrations of vapour. Takarada et al.
(1997) and Volchkov,
Terekhov and Terekhov (2004) theoretically investigated the
problem of condensation
of humid air by solving integral boundary layer equations of
energy and diffusion and
using analogy between heat and mass transfer processes.
Terekhov, Terekhov and Sharov (1998) presented a computational
analysis of combined
heat and mass transfer in condensation of water vapour from
moist air on a wall.
Desrayaud and Lauriat (2001) studied condensation of water
vapour from humid air in a
vertical channel. They derived a new correlation for the latent
and sensible Nusselt
-
Chapter 2: Literature review 13
number and demonstrated the heat and mass transfer analogy
between the sensible
Nusselt number and Sherwood number.
The problem of condensation of vapours on a vertical fin in the
presence of non-
condensable gas making use of the analogy between heat and mass
transfer was tackled
by Sarma et al. (2001).
Oh and Revankar (2006) developed a pure steam condensation model
and an
empiricism-free or minimum empirical mechanistic model for the
vapour/non-
condensable mixture condensation. In the model for the mixture,
they used general
momentum, heat and mass transport relations derived using an
analytic method, and
considered the surface suction effect. They found from
experiments that the
condensation HTC decreases with the presence of a
non-condensable gas. They also
developed a theoretical model for the annular film-wise
condensation with non-
condensable gas using heat and mass transfer analogy.
Groff, Ormiston and Soliman (2007) presented a numerical
solution of film
condensation from vapour-gas mixtures in vertical tubes, and
solved the complete
parabolic governing equations in both the liquid and gas phases,
with no need for
additional correlation equations for interfacial heat and mass
transfer.
More recently, Lee and Kim (2008) developed a theoretical model
by improving
Siddique et al.’s (1994) analytical model and investigated steam
condensation heat
transfer in the presence of air or nitrogen gas in a vertical
tube. Their results showed
-
Chapter 2: Literature review 14
that the effects of the non-condensable gas became weak as the
condenser tube diameter
decreased because of interfacial shear stress.
2.5. Simulation of Condensation Using CFD
Simulation of condensation using computer programs such as CFD
software has been
studied by researchers over the last 30 years. Lux et al. (1983)
presented a computer
model that could predict the performance of a condensing heat
exchanger with circular
tubes. This model ignored the effect of condensation and
diffusion processes.
Malalasekera et al. (1993) used CFD to model the heat transfer
and the flow in the
secondary heat exchanger of a condensing boiler. However, the
diffusion process of
water vapour in non-condensing gases and water-vapour
condensation were not
considered in his work.
Valencia (2004) carried out a CFD simulation for the
condensation of water vapour and
acids on the plate using a commercial code, FLUENT, and a
user-defined subroutine. A
numerical simulation using the commercial code and a simulation
based on empirical
correlations using the Engineering Equation Solver was carried
out for a two
dimensional (2D) vertical water-cooled plate. Experiments were
conducted for the
condensation of nitric acid, sulphuric acid and water vapour in
the presence of air on a
vertical water-cooled plate. The discrepancies between
experiments and simulation are
in a range of 7–25% depending on the combustion conditions and
the average surface
temperature of the plate.
-
Chapter 2: Literature review 15
Many film condensation models in the presence of non-condensable
gases were
presented by Martin-Valdepenas et al. (2005). Four models were
implemented in a CFD
code and compared with experimental data. They improved the code
for simulating the
gas mixing process in large containment buildings involving
steam.
A new mechanistic model for the prediction of condensation in
the presence of non-
condensable gas using a commercial computational fluid dynamic
code, CFX-4, was
presented by Karkoszka and Anglart (2006). The model was based
on a solution of the
conservation equation for the vapour phase and the transport
equation for the non-
condensable gas. A reasonable agreement between the calculated
and measured HTC
was obtained.
Ambrosini et al. (2008) used the commercial CFD code FLUENT® for
the numerical
simulation of condensation of an air-steam mixture on a flat
wall in a vertical square
channel. They divided the problem into two steps: step 0 was to
model the condensation
on an isothermal flat plate and step 1 was to model the
condensation in the CONAN
Facility (at the Università di Pisa, Italy). The condenser wall
and the coolant channel
were modelled using conjugate heat transfer and secondary
coolant was replaced by
using equivalent heat transfer conductance. Ambrosini et al.
(2008) found that the
model of the condensation on an isothermal flat plate agreed
reasonably well with
existing correlations, but that the CFD model of the
condensation in step 1 showed
larger errors in condensation rates and heat flux, especially
from near the entrance of the
water-vapour-gas mixture.
-
Chapter 2: Literature review 16
Benelmir, Mokraoui and Souayed (2009) performed a simulation of
water-vapour
condensation in the presence of non-condensable gas between two
vertical plane plates
and in a plate fin-and-tube heat exchanger in a stationary mode
using FLUENT
software. They found that the condensation rate and the HTC
increase with the inlet
velocity of the mixture.
2.6. Conclusion
Polymer compact heat exchangers have been applied successfully
in many applications
over the years. They are becoming increasingly prevalent in the
industry due to
advances in the development of polymers and a better
understanding of their properties.
In this research, experiments and simulation of heat transfer in
a counter-flow
polypropylene compact heat exchanger are conducted and analysed.
Here, the compact
heat exchanger is designed to reject waste heat by using ambient
air to cool warm water,
which can be used as an alternative for cooling towers and metal
heat exchangers.
Simulation of the heat transfer in the heat exchanger is
performed using FLUENT® to
obtain the heat rejected by the heat exchanger, and the results
are compared with the
experimental data to validate the model.
The literature review revealed that theoretical and
computational predictions of vapour
condensation and heat transfer in the presence of
non-condensable gas have focused on
the gas-and-vapour mixture. The cooling of the gas-vapour
mixture is usually calculated
by assuming a constant wall temperature or a constant heat flux
at the wall. In
condensers, this wall temperature or heat flux at the wall is in
general not known a
-
Chapter 2: Literature review 17
priori, and the temperature of the cooling fluid (for example,
water) has normally been
used as an approximation for the wall temperature. This may be a
valid approximation
when the mass flow rate of the cooling water is much larger than
that of the gas-vapour
mixture or when the mass fraction of the water vapour in the
gas-vapour mixture is low.
However, a better approach is to solve the heat and mass balance
on the cooling water
and the gas-vapour steams simultaneously.
In this study, we derive equations for the heat and mass balance
of a heat exchanger
involving water-vapour condensation in gas-vapour mixture flows
with water as cooling
fluid. The equations, in combination with many theoretical
models for heat and mass
transfer, are solved numerically. CFD software FLUENT® is used
to simulate
condensation in the heat exchanger for a more realistic model.
The predictions are
compared with available experimental results where experimental
details can be
accessed from the literature.
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 18
CHAPTER 3: EXPERIMENTAL AND
NUMERICAL STUDY OF HEAT
TRANSFER IN COMPACT HEAT
EXCHANGERS
3.1. Introduction
Many of the available and commercially used heat exchangers are
made from metals
such as aluminium and stainless steel. Although metals are good
conductor of heat, it
also brings disadvantages in terms of cost, weight, corrosion
and fouling. This has
promoted the need to develop alternative heat exchangers made
from polymer materials.
The use of polymer heat exchangers offers substantial cost and
weight savings, and
polymers resist corrosion and fouling, giving them the advantage
over metal heat
exchangers.
The objective of this chapter is to introduce an experimental
and numerical study of heat
transfer in a new counter-flow compact-plate heat exchanger made
of polypropylene.
Experiments on heat transfer in the compact heat exchanger are
conducted under
various conditions for water-to-air heat exchange. Heat
rejection rates in the
polypropylene compact heat exchanger are obtained for various
test conditions. The
effect of inlet conditions such as water flow rates and air mass
flow rates are discussed.
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 19
To validate the experimental results, a three-dimensional (3D)
simulation using CFD
software FLUENT® is performed for the heat exchanger.
3.2. The Compact Heat Exchanger
The plate heat exchanger is shown schematically in Figure 3.1
and a photograph of the
compact heat exchanger is given in Figure 3.2. It is a
counter-flow parallel-plate heat
exchanger made of polypropylene. The heat exchanger consists of
250 parallel
polypropylene flute boards sealed together, producing 249 narrow
parallel-flow
channels. The gap between the two neighbouring boards is 0.8 mm.
Hot water flows
through the gaps. Each narrow channel contains an obstacle of
the same material with a
dimension of 225 mm × 20 mm × 0.6 mm to change the flow pattern
to achieve uniform
distribution of the flow inside the channels and enhance heat
transfer. The plates have a
height and width of 609 mm each. The total available heat
exchange area of the polymer
compact heat exchanger is 184.70 m2. Polypropylene fluted boards
are used as the
material for constructing the heat exchanger. Each board has two
thin sheets that
become the plates and the plates are jointed together by the
fluted walls. Each board of
609 mm wide has 227 small square fluted channels. These channels
are mainly for
holding the thin sheet together and provide support so that the
board can withstand
some pressure.
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 20
Figure 3.1: Schematic of the plate heat exchanger.
Hot water inlet
Water outlet
Hot water inlet
Water outlet
Air channel
Air outlet
Air inlet
609 mm 609 mm
227 channels each plate
Hot water channel
Flow distributer
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 21
Figure 3.2: Photograph of the compact-plate heat exchanger.
3.2.1. Experimental Setup
Experiments of heat transfer in the compact-plate heat exchanger
were conducted under
various conditions for water-to-air heat exchange. The
experiments were conducted at
Victoria University laboratory and Yallourn power station. The
Yallourn power station
has three cooling towers, water from the towers cools the steam
coming from the
turbine so it can be pumped back to the boilers to be reheated
to steam and again goes to
Aluminium frame
Air channels
Water channels
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 22
drive the turbine. The overall experimental setup is shown in
Figure 3.3. The
experimental setup consisted of the compact-plate heat
exchanger, hot water supply
system, air supply system and measurement system.
Figure 3.3: Photograph of the experimental setup for
water-to-air heat exchanger
test.
3.2.2. Water and Air Supply System
During the experiments at Victoria University laboratory, the
hot water was supplied by
a BOSCH 32Q hot water system, which can provide 32 l/min of hot
water with a
temperature range of 37–80°C. A XF 192S pump from DAVEY, as
shown in Figure
3.4, was used to circulate the water from a bucket under the
compact heat exchanger
through the hot water system and then to the compact heat
exchanger. This construction
Compact heat exchanger Fan Water manifold Air manifold
Air outlet
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 23
method has the advantage of keeping the inlet pressure of the
hot water into the heat
exchanger at less than 30 kPa to eliminate the possibility of
water leaking in the heat
exchanger. At the experiments conducted at Yallourn power
station, the hot water was
taken from the condensate of the power plant direct to the heat
exchanger. Manifolds at
the inlet and exit of the heat exchanger were used to distribute
the water. A valve before
the inlet of the heat exchanger was used to adjust the desired
inlet flow rate of the water.
Figure 3.4: XF 192S pump for circulating water.
An electrical fan with a power of 0.37 kW, which was attached at
the exit of the air
manifold, was used to provide a uniform air stream through the
compact heat exchanger
(see Figure 3.1). A filter was installed in front of the heat
exchanger to remove dust
from the air. The fan speed can be adjusted by a speed
controller as shown in Figure
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 24
3.5, with a range of 10–50 Hz. The air flows through the air
channels in the compact
heat exchanger in counter direction with the hot water, and then
flows out the
experimental rig via a flow duct installed after the fan.
Figure 3.5: Speed controller.
3.2.3. Measurement System
Figure 3.6 shows a photograph of the data acquisition system
that was used to collect
the experimental data during the test. The hardware used was
DI-718BX Series and the
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 25
software used was WinDaq ®. All the measurements of
temperatures, flow rates and
pressure were read by the data acquisition system and stored on
a computer hard drive.
Figure 3.6: The data acquisition system.
Thermocouples, flow meters and pressure transmitters were used
to measure the air and
hot water temperatures, hot water flow rate and the pressure
differences of the air,
respectively. Air and water temperatures were measured using
K-type thermocouples,
which were inserted at the inlet and outlet of the air and hot
water flow hoses. The
thermocouples were calibrated against a mercury thermometer by
using ice and boiling
water to specify the low and high calibration values of the
thermocouples and then
applying them to the data acquisition software. The
thermocouples were also calibrated
after the tests using a secondary standard thermometer with an
accuracy of 0.1°C. The
flow rate of the hot water was measured with a flow meter (type
RS 257-133). The
flow-rate measurements were accurate to 0.0075 l/s.
-
Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 26
A pressure regulator was also installed to control the pressure
of the water at the inlet of
the heat exchanger. A differential pressure transmitter (model
FCO332) was used to
measure the pressure differences of the air across the compact
heat exchanger. The
accuracy of the pressure transmitter was 0.25% of reading value.
The pressure drop on
the inlet air nozzle was measured with a pressure sensor
(164PC01D37) to obtain the
airflow rate through the test section. The pressure sensor was
calibrated against a high-
precision manometer with an accuracy of 0.1 Pa. The atmospheric
pressure was
determined using a barometer mounted near the experimental
apparatus to determine the
density of the air passing through the nozzle.
3.3. Experiment Procedure
First, the experiments of water-to-air heat exchange in the
polypropylene compact heat
exchanger were conducted at Victoria University laboratory under
the conditions of
various airflow rates and hot water flow rates. The parameters
of each test are listed in
Table 3.1. Then, experiments at Yallourn power station,
Australia, were conducted
under a wide range of conditions of inlet water temperature,
ambient air temperature
and hot water mass flow rate. The supply inlet temperature of
the water varied between
36 and 48οC. The inlet temperature of the air varied between 11
and 25οC. The flow rate
of water ranged from 0.12 to 0.19 l/s and the flow rate of air
was about 1.4 kg/s.
Appendix A shows the tables of the experimental data from the
tests at Yallourn power
station.
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 27
Table 3.1: Test runs.
Flow rate of
hot water
(l/s)
Mass flow rate
of air
(kg/s)
Inlet temperature
of water
(οC)
Inlet temperature
of air (ambient)
(οC)
0.10 0.402–0.764 50 18.2
0.15 0.400–0.744 50 20.8
0.20 0.399–0.732 50 21.7
0.25 0.393–0.723 50 18.2
3.4. Data Reduction
To obtain the mass flow rate of air entering the heat exchanger,
during the experiments
conducted at Victoria University, a special nozzle with an inner
diameter of 300 mm
was located at the inlet section of the test rig. By applying
Bernoulli’s equation, the air
velocity in the nozzle can be obtained as Engineering
ToolBox.com (2011):
2 pV CρΔ
= (3.1)
where pΔ is the pressure drop in the nozzle, ρ is the air
density and 0.94C = is the
correction factor Engineering ToolBox.com (2011).
The air density is calculated from the ideal gas law:
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 28
PRT
ρ = (3.2)
where P is the atmospheric pressure, T is the ambient
temperature and
287 R J kgK= is the gas constant.
The mass flow rate through the nozzle was obtained as the
product of the nozzle
velocity (see Equation 3.1), air density and nozzle
cross-sectional area:
2
4adm V πρ=& (3.3)
where 300 d mm= is the nozzle diameter.
The heat rejection rate can be calculated as follows:
, ,( )w pw in w out wQ m C T T= −& & (3.4)
where wm& represents the mass flow rate of water, pwC the
specific heat of water,
,in wT the inlet water temperature and ,out wT the outlet water
temperature.
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 29
3.5. CFD Simulation
The commercial CFD software FLUENT® is used in this study to
carry out the
simulation in the heat exchanger. FLUENT® is a computer program
for modelling fluid
flow and heat and mass transfer in various geometries. It
provides modelling for laminar
and turbulent fluid flow problems. Steady state and transient
analysis can also be
performed. Using FLUENT®, the velocity, temperature and pressure
distribution in the
flow field can be obtained. The usual programming of the FLUENT®
software is not
prepared for every need of the users. However, user-defined
functions (UDFs) can be
used to define boundary conditions, material properties and
source terms according to
the user’s need using C programming language.
Figure 3.7 shows the schematic representation of the inputs
required in FLUENT® to
carry out the simulation. The first requirement of the
simulation is to build the geometry
and generate the mesh using ANSYS Workbench. Next, the boundary
conditions and
any UDFs need to be used should be defined for the model. Then,
the FLUENT®
software calculates for the solutions and the results of CFD
calculation at the end of a
computational simulation can be obtained.
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 30
Figure 3.7: Schematic of the inputs required in FLUENT® and some
outputs.
In this study, the polypropylene compact-plate heat exchanger is
simulated numerically
using FLUENT®. The 3D fluid-flow distribution and the heat
transfer in the water-to-air
heat exchanger are investigated.
3.5.1. The Governing Equations
3.5.1.1. The Continuity Equation
The mass continuity equation or mass conservation equation is
(Bird et al., 2007):
( ) ( ) ( ) 0u v wt x y zρ ρ ρ ρ∂ ∂ ∂ ∂+ + + =∂ ∂ ∂ ∂
(3.5)
where t is the time, ρ is the density and u ,v ,w are the
velocity components.
FLUENT®
Software
Geometry
User-defined
functions
(UDFs)
Boundary
conditions
Heat transfer rate +
Temperature distribution
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 31
3.5.1.2. The Momentum Equation
The following equations represent the conservation of momentum
(Bird et al., 2007):
( ) ( ) ( ) ( )
xyxx
xzx x
Pu uu uv uwt x y z x x y
g Fz
ττρ ρ ρ ρ
τ ρ
∂∂∂ ∂ ∂ ∂ ∂+ + + = − + +
∂ ∂ ∂ ∂ ∂ ∂ ∂∂
+ + +∂
(3.6)
( ) ( ) ( ) ( )
yx yy
yzy y
Pv vu vv vwt x y z y x y
g Fz
τ τρ ρ ρ ρ
τρ
∂ ∂∂ ∂ ∂ ∂ ∂+ + + = − + +
∂ ∂ ∂ ∂ ∂ ∂ ∂∂
+ + +∂
(3.7)
( ) ( ) ( ) ( )
zyzx
zzz z
Pw wu wv wwt x y z z x y
g Fz
ττρ ρ ρ ρ
τ ρ
∂∂∂ ∂ ∂ ∂ ∂+ + + = − + +
∂ ∂ ∂ ∂ ∂ ∂ ∂∂
+ + +∂
(3.8)
where P is the static pressure, τ is the stress tensor and gρ
and F are the gravitational
body force and external body force, respectively.
3.5.1.3. The Energy Equation
The conservation equation of energy is (Fluent, 2010):
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 32
( ) ( ) ( ) ( )
( )x y z
E u E v E w Et x y z
T T Tk k k vg ug wgx x y y z z
ρ ρ ρ ρ
ρ
∂ ∂ ∂ ∂⎡ ⎤ ⎡ ⎤ ⎡ ⎤+ + +⎣ ⎦ ⎣ ⎦ ⎣ ⎦∂ ∂ ∂ ∂
⎛ ⎞∂ ∂ ∂ ∂ ∂ ∂⎛ ⎞ ⎛ ⎞= + + + + +⎜ ⎟⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠⎝
⎠
(3.9)
where Tkx
∂∂
represent the energy transfer due to conduction. The cases
simulated in
this chapter contain only one species and no source terms are
used.
In Equation 3.9:
2
2P vE hρ
= − + (3.10)
where h is the sensible enthalpy and can be expressed as:
ref
T
pT
h C dT= ∫ (3.11)
where refT is 298.73 K.
For the experimental conditions simulated here, the energy
equation reduces to:
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 33
( ) ( ) ( ) ( )
2 2 2
2 2 2 ( )x y z
E u E v E w Et x y z
T T Tk vg ug wgx y z
ρ ρ ρ ρ
ρ
∂ ∂ ∂ ∂⎡ ⎤ ⎡ ⎤ ⎡ ⎤+ + +⎣ ⎦ ⎣ ⎦ ⎣ ⎦∂ ∂ ∂ ∂
⎛ ⎞∂ ∂ ∂= + + + + +⎜ ⎟∂ ∂ ∂⎝ ⎠
(3.12)
3.5.2. Geometry Creation
Initially, the 3D geometry was created for the heat exchanger
using ANSYS Design
Modeller 12.0. Due to computer power limitations, conducting
numerical study of the
complete compact heat exchanger is complicated. As a result, the
3D flow through the
single narrow water channel between two parallel airflow
channels for only one-half of
the domain shown in Figure 3.8 was simulated, because the flow
was expected to be
symmetrical about the centreline of the geometry. Due to the
complexity of generating
the flute walls that hold the two surfaces of the polypropylene
board together, the
simulation was carried out without them and the difference in
heat transfer result is
expected to be small. Figure 3.9 shows the current domain used
in this study, the
physical dimensions of the heat exchanger and the boundary
conditions used to carry
out the simulations. The domain is 609 mm in height, 304.5 mm
wide and 3.8 mm deep.
Figure 3.8: Plate heat exchange geometry.
Symmetry line
Water channel 0.6mm flow distributor
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 34
Figure 3.9: Domain dimensions and boundary conditions used in
this study.
Velocity and temperature
inlet boundary for water flow
Velocity and temperature inlet
boundary for airflow
Symmetry boundary
Pressure outlet boundary
Periodic boundary
304.5 mm
609 mm
225 mm 20 mm
50 mm
20 mm
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 35
3.5.3. Boundary Conditions
Boundary conditions specify the flow and thermal variables on
the boundaries of the
physical domain. Figure 3.9 shows the boundary conditions
utilised in this study, which
are as follows.
3.5.3.1. Inlet Boundary Conditions
The velocity and temperature values are specified at the inlet
of the water and air
channels. The direction of the inlet velocity is normal to the
surface. The inlet velocity
was calculated from channel mass flow rate and channel inlet
cross-sectional area.
3.5.3.2. Outlet Boundary Conditions
At the outlet, the pressure boundary condition is specified as a
constant value equal to
zero gauge pressure. This is the case for both the water and air
outlet channels.
3.5.3.3. Wall Boundary Conditions
Wall boundary condition is used to bound fluid and solid
regions. Since the wall zone
here is a two-side wall, which is a wall that forms the
interface between two regions
such as the fluid/solid interface, FLUENT® enables the two sides
of the wall to be
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 36
coupled, prompting the solver to calculate the heat transfer
directly from the solution in
the adjacent cells.
3.5.3.4. Periodic Boundary Conditions
The computational domain of the heat exchanger was reduced to
two halved air
channels and one water channel, and the flat boundaries of the
outer half air channel
were treated as periodic surfaces to represent the complete heat
exchanger. The periodic
boundary condition was not specified for the water channel
because one of the walls in
the water channel had a flow distributor. With this periodic
boundary condition, only
flows in one water channel and one full air channel are
simulated. The total heat transfer
from a heat exchanger with 250 channels of water and airflows
can be obtained by
timing this number of channels to the results obtained from one
channel simulation
using period boundary conditions. This periodic boundary
condition has greatly
reduced the computation cost in this simulation.
3.5.3.5. Symmetry Boundary Conditions
Symmetric boundary conditions were chosen for one-half of the
heat exchanger. This
was done because the geometry of the heat exchanger and the
expected pattern of the
flow and the thermal solution are symmetric (see Figure 3.8).
When using this type of
boundary condition in such regions, there is no need for
additional boundaries.
FLUENT® assumes zero flux of all quantities across asymmetric
boundary. There is no
convective flux across asymmetry plane, and the normal velocity
component is zero.
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 37
3.5.4. Grid Independence and Meshing
Prior to analysing the heat exchanger model in the FLUENT®
solver, it was necessary to
create a mesh structure. The mesh structure specifies the
resolution at which FLUENT®
analyses the model. Therefore, a grid independence study was
carried out to ensure the
results accuracy. Figure 3.10 presents the variation of heat
rejection rate against the grid
numbers. As shown in Figure 3.10, at stage A the results vary
with grid resolution; at
stage B the results tend towards constant, so the grid at stage
B shows grid independent.
For final simulations, the mesh was created for the heat
exchanger using quadrilateral
mesh with around 500,000 cells to ensure the simulated results
are grid independent.
25.06
25.08
25.1
25.12
25.14
25.16
25.18
0 100000 200000 300000 400000 500000 600000 700000
Hea
t Rej
ectio
n R
ate (
kW)
Number of Grid
A B
Figure 3.10: Grid independence test.
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 38
3.5.5. Working Fluid Properties
Water and air were taken as the working fluids. Viscosity,
thermal conductivity and the
density of water and air were taken as a function of
temperature. Algebraic equations
derived by the method of curve fitting are shown in Appendix B.
These equations were
programmed using UDFs and compiled in FLUENT® to compute the
properties at each
iteration of the solution process. Thus, when solving the energy
equation, the properties
changed as the temperature changed in the flow channels.
3.5.6. Solid Properties
A solid zone is a group of cells for which the heat conduction
is solved. The only
required inputs for the solid zone are the material properties
of the solid, which are
entered into the material conditions panel of the FLUENT®. In
this study, the material
used was Polypropylene and the properties are summarised as
follows:
Density ( ) 3950 Kg mρ =
Specific Heat ( ) ( )1600 pC J KgK=
Thermal Conductivity ( ) ( )0.16 k W mK=
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Chapter 3: Experimental and numerical study of heat transfer in
compact heat exchanger 39
3.5.7. Solver
The segregate solver was used to solve the governing equations
for the conservation of
mass, momentum and energy equations.